The present invention relates to a process for preparing copolymers of certain oxazolines.
It has long been known to produce 2-(C.sub.1 to C.sub.3 alkyl)-.DELTA..sup.2 -oxazoline copolymers such as those which are prepared by the process of this application (hereinafter called polyoxazolines) (Chem. Reviews 71 [1971] 496).
In order to use these polyoxazolines on an industrial scale they must be manufacturable with reproducible properties. The viscosity number is one such important property for many applications. The viscosity number, also called the J value, is defined herein under the following test conditions: 25.degree. C.; 0.5% of the polyoxazoline (weight/vol of solution) in a mixture of isopropanol/water (50%-volume of isopropanol).
Polyoxazolines with J values in the range from 35 to 70 ml/g have been obtained, for instance, in the absence of solvents by heating for 15 hours at 170.degree. C. in the final phase of a polymerization (German Patent 12 63 300). The products are characterized by a more or less pronounced degree of branching (J. Macromol. Sci. A9 [1975] 703 ff). Accordingly, they are not optimally suited for many applications, in particular the use not yet part of the state of the art which is disclosed in German patent applications P 30 36 127.4-44 and P 30 36 101.4, corresponding to U.S. application Ser. No. 301,270, filed Sept. 11, 1981, whose disclosures are incorporated by reference herein.
Similar products have been obtained by polymerizing at temperatures approximately up to 200.degree. C. for polymerization times that were substantially shortened.
Again, polyoxazolines within the stated viscosity range have been obtained by inducing a planned branching or partial crosslinking by addition of a slight amount of an alkylene- or arylene-bis-oxazoline as a comonomer to the polymerization input material (Angew. Chem. 78 [1966] 920, right-hand column 5th paragraph). These products are not optimally suited for many applications, in particular for the above cited use not yet in the state of the art.
Another drawback of the state of the art of preparing these copolymers, especially as regards the above cited use not yet in the state of the art, is that attainment of reproducible J values from batch to batch is difficult.
It is known that polyoxazolines can be produced in the presence of inert, organic solvents. Polyoxazolines so prepared however do not simultaneously meet the criteria of "linearity" and "viscosity number" as required for the polymers prepared by this invention.
It is, moreover, known that cation-active catalysts whose anions are strongly nucleophilic act in a chainbreaking manner. Accordingly, as a rule, those catalysts are used whose anions possess only a slightly nucleophilic character. It is further known that nucleophilic impurities in the polymerization batch, for instance water, amines and alcohols, act in a chain-breaking manner. Thus, the production of high J value polyoxazolines is made difficult or even impossible. The purification of the monomers, therefore, is considered necessary. Monomers have been treated with acid chlorides or acid anhydrides and have been fractionally distilled (U.S. Pat. No. 3,326,929).
The drawbacks consequently incurred include more than insignificant losses in monomer. Moreover, the monomers so purified were not free from newly entrained, chain-breaking impurities.
It has already been proposed to treat the monomers with diisocyanates and then to fractionally distill them. (Angew. Chem. 78 [1966], 920, right-hand column, 2nd paragraph). This purification procedure, however, has not been generally successful with the 2-(C.sub.1 to C.sub.3 alkyl)-.DELTA..sup.2 -oxazolines because these are not inert with respect to such isocyanates, except for 2-isopropyl-.DELTA..sup.2 -oxazoline. When producing the polyoxazolines on an industrial scale, as a rule, a fractional distillation in the absence of entraining agents is the accepted procedure for purifying the monomers (Angew. Chem. 78 [1966], 920, right-hand column, 2nd paragraphy). However, the above-described drawbacks essentially still remain.